The invention relates to a transparent, biaxially oriented polyester film composed of a layer which comprises at least 80% by weight of thermoplastic polyester. The invention further relates to a process for producing the film and to the use of the film.
The polyester film of the invention also comprises a specific pigment system which provides improved winding of the film and gives a highly transparent film with high-gloss surfaces. The film has very high suitability for use as a packaging film, in particular for metallizing or ceramic coating, and for applications in the industrial sector, e.g. as a substrate for stamping films.
In the packaging industry there is a high level of demand for highly transparent and high-gloss plastic films, e.g. biaxially oriented polypropylene films or biaxially oriented polyester films. These highly transparent, high-gloss films have a characteristic appearance and give the packaging, for example, an appearance which is particularly attractive and therefore particularly effective for promotional purposes. The problem with films subject to high requirements for the optical properties of the film is, inter alia, their windability, which does not always meet the requirements set, in particular if the machine rolls are processed to give wide customer rolls of high running length.
The prior art shows how the optical properties, in particular the gloss and the haze, of biaxially oriented polyester films can be improved.
EP 347 646 describes a biaxially oriented polyester film which has at least one outer layer (A) which comprises a concentration of from 0.5 to 50% of a filler, the diameter of this filler being in a particular ratio to the thickness of the outer layer. The outer layer also has a certain thickness and a certain degree of crystallization, which is determined with the aid of Raman spectroscopy. The topography of the outer layer A makes the film particularly suitable for magnetic recording tapes. The specification gives no information concerning the transparency of the film or the gloss achieved on the outer layer A. A film produced as in EP-A 347 646, did not have the desired transparency, nor the desired glossy surface. The gloss of this surface and the haze of the film are outside the range claimed in this specification.
EP-A-0 514 129 describes a transparent multilayer film which encompasses a primary layer substrate made from polymer material which has, at least on one of its surfaces, a secondary layer made from polymer material and having glass beads and silicon dioxide particles at particular concentrations and in particular size distributions. The arrangement of the secondary layer may be on one side or on both sides of the primary layer substrate. The film improves haze and processing properties, but the specification teaches nothing concerning improvement of the gloss and of the windability of the film.
EP-A-0 604 057 describes a transparent multilayer film which encompasses a primary layer substrate made from polymer material and substantially free from fillers, and having, on one of its surfaces, a secondary layer which is made from polymer material and which comprises a concentration of from 100 to 1000 ppm of silicone resin with a median particle diameter of from 1.5 to 12.5 μm. A disadvantage of the silicone particles is that these are comparatively expensive and do not represent an acceptable solution for the packaging market. In addition, films equipped with pigments of this type tend to telescope on winding. Nor does that specification give any indication at all as to how the topography of such a film is to be adjusted for simultaneous improvement of gloss and windability.
DE-A-16 94 404 describes a laminate with a plurality of layers of an oriented crystalizable thermoplastic film, in which at least one of the outer layers comprises an additive. The additives are conventional inert inorganic or organic particles which, in the case of the inert particles such as SiO2, are added at a concentration of from 1 to 25% by weight to the outer layers. The particle size here is from 2 to 20 μm. The laminates may be metalized with aluminum for decorative purposes or used for magnetic tapes. Although the teaching of this specification enables the processing properties and the haze of the film to be improved, the specification gives no teaching concerning improvement in the gloss or in the windability of the film.
EP-A-0 502 745 describes a coextruded biaxially oriented polyester film with at least three layers, of which an outer layer A                a) comprises inorganic particles with a mean primary particle size D in the range from 1 to 100 nm and complying with the equation D<T<200D, where T is the thickness of layer A,        b) comprises particles B with a mean primary particle size D1 in the range from 0.3 to 2 μm and with a primary particle size distribution having a relative standard deviation of not more than 0.6, and        c) where the mean primary particle size D of the particles A is smaller than the mean primary particle size D1 of the particles B.        
The teaching of this specification in particular improves the processing performance of the film. The specification gives no teaching concerning improvement of the gloss of the film, or of its haze or windability.
It was therefore an object of the present invention to provide a biaxially oriented polyester film which has very good optical properties and in particular has very high gloss and very low haze. The film should also be very easy to wind, in particular when the machine roll is processed to give wide customer rolls of high running length. In addition, the film is to be very easy to produce and to process, and is to provide a very good oxygen barrier after metalization or after coating with oxidic materials. In summary, the object was to provide a film with the following combination of features:                high gloss        low haze        good winding        low oxygen transmission of the film after metalizing or after coating with oxidic materials        low coefficients of friction.        
The gloss of the film is intended to be greater than 150, and the haze lower than 4.0%. The film is intended to be very easy to wind, and this means in particular that there are to all intents and purposes no longitudinal corrugations in customer rolls produced from a machine roll. This applies in particular when these are wide customer rolls of high running length. It is intended that less than 1.0 cm3 of oxygen per square meter and per day will diffuse through the metalized film when it is subjected to air at a pressure of 1 bar. In respect of other properties, the quality of the film is intended to be at least equal to that of the known packaging films of this type. The film is also intended to be simple and cost-effective to produce, and to be very easy to process on conventional machinery. The coefficient of friction on both surfaces is intended to be less than 0.6.
A further intention is to ensure that cut material arising during film production can be reintroduced to the manufacturing process as regrind in amounts of up to 60% by weight, based on the total weight of the film, without any resultant significant adverse effect on the physical or optical properties of the film.